The most common method for production of ethanol by fermentation of sugars utilizes strains of Saccharomyces cerevisiae yeast. The predominant product of such fermentations is ethanol and selectivity can reach over 99%, but other byproducts can be formed. The process of byproduct formation by yeast has recently been reviewed (Hazelwood et al., 2008). The main byproducts are called “fusel oils” or “fusel alcohols” and they consist of higher carbon number alcohols such as n-propanol, isobutanol, isoamyl alcohol, and optically active amyl alcohol. The predominant fusel alcohol is isoamyl alcohol.
There is a market for fusel alcohols and their esters in solvents, flavors and fragrances. These applications generally are for lower volumes than for fuel ethanol and represent an economically important alternative to simply leaving them in the final fuel alcohol.
During the distillation of ethanol containing mashes, there is an accumulation of the fusel alcohols on specific trays of the column (Guymon, 1958). Although, the overall amount of fusel alcohols may not exceed 1%, there are points in the column where they can be as high as 15%. The conventional approach to recovery of fusel alcohols is the use of decantation, which is accomplished by the addition of large volumes of water to cause a phase separation with the upper, lower density phase being enriched with the fusel alcohols (Guymon, 1958). The main problem with this approach is that the water rich phase contains a significant amount of ethanol and must be returned to the distillation column for recovery. The large volume of added water, usually twice as much water by volume is added to the initial fusel alcohol rich stream coming from the column, creates a large energy load on the process. This added water must be removed from the resulting mixture at significant cost. Furthermore, the added volume is also a problem causing a bottleneck in the process.
Reactive distillation has been shown to be effective in the recovery of fusel alcohol esters (Kucuk and Ceylan, 1998; Saha et al., 2005). In this process the fusel alcohols are converted to esters in the presence of a solid acid catalyst, such an ion exchange resin, in a distillation column. The esterification reaction is an equilibrium reaction,RCO2H+R′OH⇄RCO2R′+H2Owherein it can be seen that water promotes the reverse reaction, inhibiting ester formation. This factor makes it preferable to have a low water content in the reaction mixture. A typical fusel alcohol containing stream from an ethanol distillation column can contain as much as 40% water, which would significantly impede esterification.
To summarize, there are two main obstacles that need to be addressed. A first objective is to efficiently recover fusel alcohols from an ethanol plant distillation column without introducing large quantities of water to the system. A second objective is to exclude as much water as possible from a reaction mixture for production of fusel alcohol esters.